When you have a bucket of lava and dump it on a single block of water that isn't touching any other blocks of water or flowing in any direction then it deletes the block and replaces it with lava instead of turning it into obsidian. This also happens with dumping water onto lava.
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Duplicate of MC-8930, please use the search function to see if your bug has already been submitted. Currently over 52% of tickets are being closed as duplicate.
I did use the search function but I didn't see anything in the title of that one that made it look like mine.
Confirmed in 13w09b.